Patent GB-1396691-A (GB)
Title: Positioning device using photoelectric scanning
IPC: G03F9/00
Filed: 1972-05-17
Abstract: 1396691 Optical postion finders CANON KK 17 May 1972 [17 May 1971 2 March 1972] 23246/72 Heading H4D A system for determining the position of one object relative to another includes optically superposing similar indices on the two objects and using photo-sensitive detector means to provide signals representing the deviation of the indices. In a system for positioning a mask 16 relative to a semi-conductor chip 11, a lamp 17 illuminates a circular mark 11A on the chip 11, through an aperture 16 2 . The images of mark and aperture are superposed by semireflecting mirror 19 on to a detector plate 21 which carries four detectors situated to receive light reflected from the chip surface outside the mark which is smaller than the aperture (Fig. 3 not shown). If the mark is off-centre the outputs from opposed pairs of detectors, i.e. 21A, 21C and 21B, 21D are unbalanced. Correcting signals are applied to servomotors 12X, 12Y. In a modification (Fig. 5, not shown) the image is rotated using a Dove prism and a single detector is used. The detector output is compared in phase to quadrature reference signals to provide drives for the servomotors. Similar arrangements are described (Fig. 13, not shown), in which the image is stationary and is scanned by a rotating prism or disc with a slot or by a rotating bundle of optical fibres. To compensate for variations in reflectivity of the reference mark, or in off-centring of the rotating members, the mark is provided with radial arms (Figs. 9-10, not shown), which pulse modulate the detector output. Band pass filtering is used to remove any low frequency error signals. In a further embodiment, the reference mark takes a "cartwheel" form (Fig. 21). This is projected on to a circular mark on the semiconductor chip as before. The superposed images are scanned by a rotating disc with a slot. The positioning takes two stages. First the mask bearing the "cartwheel" mark is positioned relative to the scanning disc by utilizing the frequency modulation of the reflected light caused by de-centring the "cartwheel" mark. Then the chip is positioned using the circular mark thereon. For more accurate positioning the circular mark is replaced by a "star" having a different number of "rays" than the "cartwheel" has spokes. Thus two different frequency modulations occur, centred on discrete frequencies (Figs. 29-30, not shown). In a third embodiment Fig. 31, the chip W #  is first centred approximately using the central circular portion of the reference mark P R . During this stage the contrast of the mark against its background is also determined in order to provide the correct phase of reference signals for later measurements. When the initial stage is completed, the error signals generated by the scanning disc 1019 having a frequency component equal to the angular speed of the disc are approximately zero. This condition is detected and used to automatically trigger the fine stage where the deviation of the chip is determined by frequency demodulation as above from the notched edge of the reference mark P R  and the deviation of the mask 1009 is determined from the notched edge of mark 1011. When the chip is correctly positioned filter 1014 is removed and light source 1013 illuminates the photo-resist on the chip through mask pattern 1010.